One gives advice; diet or ail badly (9)
I believe the answer is:
editorial
'one gives advice' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe there's a link between them I don't understand?
'diet or ail badly' is the wordplay.
'badly' indicates anagramming the letters (letters in the wrong order).
'diet'+'or'+'ail'='dietorail'
'dietorail' is an anagram of 'EDITORIAL'.
Can you help me to learn more?
(Other definitions for editorial that I've seen before include "matter of opinion" , "paper item" , "Statement of a newspaper's opinion" , "(Paper's) leader" , "Article written" .)